In a current wireless communication system, for example in a 3G/4G wireless communication system, in order to achieve a high peak rate, the 3G/4G system usually adopts a typical configuration of a high order modulation before a Turbo decoding, such as a High Speed Packet Access+ (HSPA+) system.
In many communication systems, for example in a Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) system, a Wideband CDMA (WCDMA) system and a Long Term Evolution (LTE) system, 64Quadrature Amplitude Modulation (QAM) high order modulation may be adopted. 64QAM may achieve a high transmission rate without the need to increase a bandwidth in the wireless system. However, the high order modulation may only be achieved in a high Signal to Noise Ratio (SNR) environment.
It should be noted that for the 64QAM, each modulation symbol has a corresponding transmission state. In the prior arts, for the 64QAM signals received by a receiver in the wireless communication system, there are three methods to calculate a constellation demodulation soft bit.
The first method adopts a Logarithm Likelihood Ratio (LLR) method to calculate each bit metric of a channel symbol:
      Λ    k    =      log    ⁢                                        ∑                          j              =                                                1                  ⁢                                      :                                    ⁢                                      a                    n                                                  =                1                                      M                    ⁢                                          ⁢                      exp            ⁢                          {                              -                                                      1                                          2                      ⁢                                              σ                        n                        2                                                                              ⁡                                      [                                                                                            (                                                                                    r                              k                                                              (                                i                                )                                                                                      -                                                          x                              ⁡                                                              (                                j                                )                                                                                                              )                                                2                                            +                                                                        (                                                                                    r                              k                                                              (                                q                                )                                                                                      -                                                          y                              ⁡                                                              (                                j                                )                                                                                                              )                                                2                                                              ]                                                              }                                                            ∑                          j              =                                                1                  ⁢                                      :                                    ⁢                                      a                    n                                                  =                0                                      M                    ⁢                                          ⁢                      exp            ⁢                          {                              -                                                      1                                          2                      ⁢                                              σ                        n                        2                                                                              ⁡                                      [                                                                                            (                                                                                    r                              k                                                              (                                i                                )                                                                                      -                                                          x                              ⁡                                                              (                                j                                )                                                                                                              )                                                2                                            +                                                                        (                                                                                    r                              k                                                              (                                q                                )                                                                                      -                                                          y                              ⁡                                                              (                                j                                )                                                                                                              )                                                2                                                              ]                                                              }                                          .      x(j),y(i) are an I part and a Q part of a transmission signal respectively, r(j)k, r(q)k are an I part and a Q part of a received signal respectively, an is any one from six of [a1, a2, a3, a4, a5, a6] and m=26.
The second method is to calculate the numerator and the denominator in the first method by using a maximum function. Therefore, the bit metric of bit position is calculated as follows:
                    Λ        k            =              log        ⁢                                            MAX                              j                =                                                      1                    ⁢                                          :                                        ⁢                                          a                      n                                                        =                  1                                            M                        ⁢                                                  [                          exp              ⁢                              {                                  -                                                            1                                              2                        ⁢                                                  σ                          n                          2                                                                                      ⁡                                          [                                                                                                    (                                                                                          r                                k                                                                  (                                  i                                  )                                                                                            -                                                              x                                ⁡                                                                  (                                  j                                  )                                                                                                                      )                                                    2                                                +                                                                              (                                                                                          r                                k                                                                  (                                  q                                  )                                                                                            -                                                              y                                ⁡                                                                  (                                  j                                  )                                                                                                                      )                                                    2                                                                    ]                                                                      }                                      ]                                              MAX                              j                =                                                      1                    ⁢                                          :                                        ⁢                                          a                      n                                                        =                  0                                            M                        ⁢                                                  [                          exp              ⁢                              {                                  -                                                            1                                              2                        ⁢                                                  σ                          n                          2                                                                                      ⁡                                          [                                                                                                    (                                                                                          r                                k                                                                  (                                  i                                  )                                                                                            -                                                              x                                ⁡                                                                  (                                  j                                  )                                                                                                                      )                                                    2                                                +                                                                              (                                                                                          r                                k                                                                  (                                  q                                  )                                                                                            -                                                              y                                ⁡                                                                  (                                  j                                  )                                                                                                                      )                                                    2                                                                    ]                                                                      }                                      ]                                ,    and              Λ      k        =                            MAX                      j            =                                          1                ⁢                                  :                                ⁢                                  a                  n                                            =              1                                M                ⁡                  [                      {                          -                                                1                                      2                    ⁢                                          σ                      n                      2                                                                      ⁡                                  [                                                                                    (                                                                              r                            k                                                          (                              i                              )                                                                                -                                                      x                            ⁡                                                          (                              j                              )                                                                                                      )                                            2                                        +                                                                  (                                                                              r                            k                                                          (                              q                              )                                                                                -                                                      y                            ⁡                                                          (                              j                              )                                                                                                      )                                            2                                                        ]                                                      }                    ]                    -                                    MAX                          j              =                                                1                  ⁢                                      :                                    ⁢                                      a                    n                                                  =                1                                      M                    ⁡                      [                          {                              -                                                      1                                          2                      ⁢                                              σ                        n                        2                                                                              ⁡                                      [                                                                                            (                                                                                    r                              k                                                              (                                i                                )                                                                                      -                                                          x                              ⁡                                                              (                                j                                )                                                                                                              )                                                2                                            +                                                                        (                                                                                    r                              k                                                              (                                q                                )                                                                                      -                                                          y                              ⁡                                                              (                                j                                )                                                                                                              )                                                2                                                              ]                                                              }                        ]                          .            
The third method is to use a distance between a value of a received signal and a constellation point as reliability information. FIG. 1 shows the 64QAM soft demodulation as follows:Λk(1)=r(q)k Λk(2)=r(i)k Λk(3)=4/√{square root over (42)}−|r(q)k|Λk(4)=4/√{square root over (42)}−|r(i)k|Λk(5)=2/√{square root over (42)}−|4/√{square root over (42)}−|r(q)k∥)Λk(6)=2/√{square root over (42)}−|4/√{square root over (42)}−|r(i)k∥).
Obviously, the first method and the second method are very complex in terms of computation but the third method only requires a simple operation. According to a simulation result, the third method provides a similar performance compared with the other two methods, thus the third method is used in practical systems.
As shown in FIG. 2, in a typical wireless receiver, for example in a wireless receiver in TD-SCDMA system, a typical 64QAM demodulation and Turbo decoding procedures are as follows.
The received signal is transformed to a baseband signal and then the baseband signal is sent to a data demodulation module to perform the 64QAM demodulation. The demodulated signal is quantized by a bit quantization module and decoded by a decoder (Turbo decoder) and the required original signal can be obtained as a result.
Λ′demap=[Λ1(1),Λ1(2),Λ1(3),Λ1(4),Λ1(5),Λ1(6),Λ2(1) . . . Λ5(2) . . . ΛN(1) . . . ΛN(6)].
Λ′demap is a demodulated soft bit. Before being input into the Turbo decoder, the data are usually quantized by a 4-bit uniform quantization or by a 5-bit uniform quantization, shown in the following tables:
Quantization Table of a 4-bit Uniform QuantizationInput (x)Outputx < −0.875−8−0.875 <= x < −0.75−7−0.75 <= x < −0.625−6−0.625 <= x < −0.5−5−0.5 <= x < −0.375−4−0.375 <= x < −0.25−3−0.25 <= x < −0.125−2−0.125 <= x < 0−10 <= x < 0.12500.125 <= x < 0.2510.25 <= x < 0.37520.375 <= x < 0.530.5 <= x < 0.62540.625 <= x < 0.7550.75 <= x < 0.87560.875 <= x7
Quantization Table of a 5-bit Uniform QuantizationInput (x)Outputx < −0.9375−16−0.9375 <= x < −0.875−15−0.875 <= x < −0.8125−14−0.8125 <= x < −0.75−13−0.75 <= x < −0.6875−12−0.6875 <= x < −0.625−11−0.625 <= x < −0.5625−10−0.5625 <= = x < −0.5−9−0.5 <= x < −0.4375−8−0.4375 <= x < −0.375−7−0.375 <= x < −0.3125−6−0.3125 <= x < −0.25−5−0.25 <= x < −0.1875−4−0.1875 <= x < −0.125−3−0.125 <= x < −0.0625−2−0.0625 <= x < 0−10 <= x < 0.062500.0625 <= x < 0.12510.125 <= x < 0.187520.1875 <= x < 0.2530.25 <= x < 0.312540.3125 <= x < 0.37550.375 <= x < 0.437560.4375 <= x < 0.570.5 <= x < 0.562580.5625 <= x < 0.62590.625 <= x < 0.6875100.6875 <= x < 0.75110.75 <= x < 0.8125120.8125 <= x < 0.875130.875 <= x < 0.9375140.9375 <= x15In the HSPA+ system, the 64QAM modulation is used. The performance deteriorates great with the 4-bit uniform quantization but deteriorates little with the 5-bit or more uniform quantization, having 2 to 3 dB performance improvement in high rate compared with the 4-bit uniform quantization. However, in practice, it is not convenient to realize the 5-bit quantization.